An X-ray analysis of galaxy NGC 3079 suggests that activity in its central region and its much larger galactic-scale superbubble unfolded on very different timescales. The nuclear superbubble has a modeled characteristic age of 0.92 million years, while the larger bubble has a modeled dynamical age of 29.6 million years. The authors view that gap as suggestive of episodic feedback, although the ages come from adopted bubble models rather than direct clocks.
A separate chemical estimate put the upper mass of stars that may have supplied the nuclear enrichment at about 13 to 15 times the mass of the Sun, if core-collapse supernovae alone produced the observed pattern. That result is a constraint from adopted supernova-yield models, not a direct census of the progenitor stars.
Reading the central spectrum
The analysis covered the central region of NGC 3079, roughly 4 kiloparsecs across, including a nuclear superbubble about 1 kiloparsec in scale. It also examined the galactic-scale bubble out to about 5 arcminutes, or 25 kiloparsecs. The spectra were fitted with SPEX using the C-statistic for X-ray data.
The researchers compared five alternative descriptions of the nuclear emission. M1 had the minimum corrected-Akaike score. M2 was statistically comparable, with a ΔAICc of 0.8, while M3 through M5 were considerably less supported. The main nuclear abundance and age estimates therefore come from M1.
The chemistry changes with the model
In M1, the model’s multi-temperature thermal component contributed about 26 percent of the observed energy flux from 0.2 to 2 keV. The charge-exchange component, or CX, contributed about 13 percent. CX was much more prominent around selected oxygen and neon features, contributing roughly 67 percent, 69 percent and 52 percent of the combined thermal and CX emission, but only 2 percent to 6 percent near an iron feature.
That distinction changes the inferred chemistry. With CX included, the best-fit oxygen-to-iron and neon-to-iron ratios were 0.47 and 0.77 in solar units. When CX was left out, the fitted ratios rose to 1.6 and 2.0. The comparison shows that the abundance pattern depends strongly on how charge exchange and temperature structure are handled.
The same M1 ratios, compared with yields weighted for the expected distribution of stellar masses, favored an upper progenitor-mass cutoff of about 13 to 15 solar masses for pure core-collapse enrichment. When Type Ia supernovae were allowed into the calculation, the permitted cutoff generally increased with their assumed share. Even for fractions up to 10 percent, however, the one-sigma upper limits remained below 25 solar masses.
A larger bubble, and an unresolved engine
The galactic-scale superbubble out to about 25 kiloparsecs was adequately described by a single-temperature model in collisional ionization equilibrium, with a nearly constant fitted temperature of 0.30 to 0.38 keV. Limited photon statistics mean that the data do not strongly test more complex temperature structure.
The energy accounting leaves the central power source unresolved. The nuclear superbubble’s energy scale did not distinguish an active galactic nucleus from a current starburst. For the larger bubble, reproducing the modeled shock required a star-formation rate of at least 17 solar masses per year under the paper’s density scaling, compared with a present rate of about 2.6 solar masses per year. The authors say that comparison favors a previous feedback event.
What the result does not settle
The findings apply to NGC 3079 and should be read as a model-based reconstruction of that galaxy. The abundance ratios change with the treatment of charge exchange and temperature, while the larger bubble’s age assumes a radius of about 5 arcminutes, or 25 kiloparsecs, within a self-similar bubble model. The study therefore does not directly measure the supernova progenitor distribution or uniquely identify the source that powered the nuclear bubble.
Paper data and sources
Original title: X-ray measurements of the elemental abundances for the diffuse emission in the nuclear starburst galaxy NGC 3079
Authors: Jiejia Liu, Junjie Mao, Shuinai Zhang et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-09-29
DOI: Not available
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